Material cycles and energy: photosynthesis

Transcription

1 7 Material cycles and energy: photosynthesis Remember: Plants are living organisms and can carry out all the life processes. Plants must be able to make foods. The foods provide raw materials for growth as well as energy. Plants do not move very much, so they must be able to feed without moving. Preliminary knowledge: Photosynthesis Very few plants can trap or catch ready-made food. Instead they must make their own food. They make their food by combining the gas carbon dioxide from the air with water from the soil. They need energy to do this and as you will remember, this energy comes from sunlight. The energy in sunlight is trapped by a green pigment, called chlorophyll, in the chloroplasts of cells in the leaves of the plant. The method plants use to feed themselves is called photosynthesis. The name gives you a clue about what happens during this process. Photo means using light and synthesis means putting together. Photosynthesis is summarised by the word equation: light energy carbon dioxide + water glucose + oxygen (reactants) chlorophyll (products) The glucose is usually converted into starch. Starch is a larger molecule, made up of many glucose units. It is a more stable and useable molecule for food and energy storage. The process of photosynthesis therefore provides food for the plant. This food can be used to provide energy that can be stored as starch or used for the growth of the plant. The food that the plant keeps and stores inside its body is called biomass. This biomass eventually provides food for many animals _Ch07.indd 87 12/03/15 8:48 PM

2 Photosynthesis Used in respiration to provide energy. Stored as starch to be used later. Glucose in plant Used for growth. For example it becomes: cellulose cell wall; and compounds in seeds and fruits. Together this makes up biomass, which can become food for animals. 7 Material cycles and energy: photosynthesis Plants and animals The biomass of plants provides animals with a useful food supply, which is why many animals eat plants. Sometimes the animal eats the whole plant; sometimes it may just steal the food store. One way or another, all animals depend on plants for their food. This will be studied in more depth in a section on food chains in Chapter 8. Balancing oxygen and carbon dioxide in the atmosphere Photosynthesis gives out the gas oxygen as one of its products. All living cells need oxygen to release the maximum amount of energy from food. This is respiration (see Chapter 3). The balance between the processes of photosynthesis and respiration maintains the constant composition of the atmosphere. Water Carbon dioxide PHOTOSYNTHESIS RESPIRATION Human activities can affect this balance. Glucose Oxygen The burning of fossil fuels uses up oxygen and produces carbon dioxide. The cutting down of forests means less photosynthesis, so carbon dioxide levels rise (see Chapter 8, Deforestation). Factors affecting photosynthesis Any food that the plant has left over, after it has used some for energy, can be used for growing. The plant will be able to grow, so long as it can photosynthesise more than it respires. The four factors that affect photosynthesis are given on the next page. Did you know? There are two other types of organism, besides plants, that contain chlorophyll and can make their own food by photosynthesis. These are: algae a large and diverse range of organisms, ranging from single-celled algae to different types of seaweed cyanobacteria sometimes called blue-green algae, despite being a type of bacteria. Both of these organisms are usually found in aquatic (water) environments and thus they are an important food source in aquatic food chains _Ch07.indd 88 12/03/15 8:48 PM

3 Light intensity: Light provides the energy needed to join carbon dioxide and water together. The more light there is, the greater the rate of photosynthesis. The level of carbon dioxide: The air must provide carbon dioxide. There is very little carbon dioxide in the normal atmosphere, so this gas must be replaced quickly by respiration. Temperature: Thermal energy is needed so that all the chemical reactions in the plant can happen quickly enough. The best temperature for photosynthesis is around 25 C. Temperatures above 40 C damage plant cells and photosynthesis comes to a halt. Water: Water is needed to combine with carbon dioxide and to carry foods around the plant s body. Water is important for photosynthesis but it is not as important as the other factors. Investigation: Factors affecting photosynthesis starch detection method Scientists are very interested in how different factors affect photosynthesis. If they can understand how plants grow, then they may be able to make plants grow more quickly. This could provide more food for humans and other animals. When a scientist has an idea or prediction that they want to check, they will need to carry out an experiment. The idea or prediction is called a hypothesis. The experiment must be reliable, or the information it gives will not be useful (see the section on Investigations in Science ). There are a number of different ways in which photosynthesis by the plant can be measured. The easiest method is to show whether or not the plant has been able to make starch. (Remember that you learned about the iodine test for starch in Chapter 2.) The diagram on the next page outlines the method for showing that light is needed for the production of starch. This method for detecting photosynthesis is qualitative, meaning you can tell whether or not photosynthesis has occurred, but not how much photosynthesis has occurred. If the scientist wanted to check how variations in temperature, light, water or carbon dioxide affected the growth of a plant, then it would be necessary to use many plants. The different tests could be carried out on different plants. 1 Describe how you would use this method to investigate the effect of temperature on photosynthesis. 2 Explain why it is important to carry out the tests at the same time on plants that were the same species and the same size at the start of the experiment. Working Scientifically Preliminary knowledge: Photosynthesis _Ch07.indd 89 12/03/15 8:48 PM

4 The carbon cycle Carbon dioxide in the air Combustion of fossil fuels releases carbon dioxide. Photosynthesis by plants traps carbon dioxide and turns it into sugars. During respiration animals convert sugars into carbon dioxide. Respiration by plants turns sugars into carbon dioxide. Feeding passes sugar to animals. Plant remains Animal remains and excretion During decomposition bacteria and fungi break down sugars into carbon dioxide. Go further The nitrogen cycle Lightning gives enough energy to combine nitrogen and oxygen from the air. Nitrogen in the air 7 Material cycles and energy: photosynthesis Plant protein Feeding Animal protein Animal and plant remains: Dead bodies and excreted waste. Decomposition: Bacteria and fungi break down proteins. Nitrates in the soil If the soil is waterlogged, some bacteria change nitrates back into nitrogen (a process called denitrification). What a waste! _Ch07.indd 96 12/03/15 8:49 PM

5 Decomposition and nutrient cycling Exercise 7.2: Plant nutrition 1 Write down two jobs carried out by the stem of a plant. 2 What is the main job of a leaf? Give two ways in which the leaf is well adapted for this job. 3 Mango trees are grown in hot, dry countries where the soil can be hard and tightly compacted. Farmers water the mango trees by spraying water onto the soil around them. (a) (i) a small amount of the water actually reaches the roots of the trees. Suggest one reason why. (ii) Suggest one other reason why mango trees do not grow well in soil that is hard and tightly packed. (b) Give two reasons why mango trees and other plants need water. (c) There is a new method of watering mango trees. Trenches are dug between the trees and filled with small pieces of rock. Plastic pipes with small holes in them are placed on top of the pieces of rock and water is pumped along the pipes. Mango trees watered by this method produce 15% more fruit. Mango trees Hard, tightly packed soil Trenches in the ground filled with pieces of rock Plastic pipe with small holes in it (i) Suggest one reason why pieces of rock are placed in the trenches under the pipes. (ii) With the new method, farmers can also add nitrates to the water in the pipes. Give one reason why plants need compounds that contain nitrogen. 4 The drawing shows a plant called Tillandsia. (a) (i) The leaves of this plant absorb light. Why do plants need light? (ii) Tillandsia plants grow on the high branches of trees in rainforests. These plants cannot grow well on the lowest branches. Explain why. (b) Tillandsia plants do not have root hairs on their roots. What two substances do most plants absorb through their root hairs? _Ch07.indd 97 12/03/15 8:49 PM

6 (c) Which diagram below shows a root hair? A B C D Extension question 5 Five sets of plants were grown. Each one had a slightly different treatment. The plants were weighed after two weeks of growth. How they were treated and how they grew is shown in this table. Working Scientifically Weight/ grams Treatment 34 Ideal conditions for air, water, light and minerals 32 Ideal conditions for air, water and light but only half minerals 18 Ideal conditions for air, water and minerals but only half light 28 Ideal conditions for water, light and minerals but only half the amount of air 19 Ideal conditions for air, light and minerals, but only half the amount of water (a) Plot the results on a bar chart. (b) Which treatment had the greatest effect on the plants growth? Explain why. 7 Material cycles and energy: photosynthesis _Ch07.indd 98 12/03/15 8:49 PM